We report the case of a 58-year-old male patient who visited our hospital for the management of edema and proteinuria. He was diagnosed as having nephrotic syndrome, with serum total protein and albumin levels of 4.6 g/dL and 2.1 g/dL, respectively, and a urinary protein excretion level of 6.0 g/day. A percutaneous renal biopsy showed features of membranous glomerulonephritis, with capillary-wall granular deposits of IgG and C3 on immunofluorescence and subepithelial immune complex deposits on electron microscopy. No other secondary cause of membranous glomerulopathy was found even after extensive investigations. The patient was started on mycophenolate mofetil (MMF) monotherapy (1,500 mg/day), and 18 months after the start of this therapy, the proteinuria decreased to 0.5 g/day, with return to a normal serum albumin level. No digestive symptoms, kidney function worsening or increase in blood pressure were noted during treatment. These findings suggest that MMF monotherapy is effective and safe for the treatment of membranous nephropathy.